The present invention relates to an improvement in an apparatus for single crystal growing by the Czochralski method. More particularly, the invention relates to an improvement in an apparatus for growing single crystals of high-purity semiconductor silicon by the Czochralski method by which high-quality single crystals of semiconductor-grade silicon can be produced with remarkably decreased costs.
As is well known, semiconductor single crystals or, in particular, single crystals of high-purity semiconductor silicon are produced either by the so-called Czochralski method or the so-called floating zone-melting method, of which the major trend in the modern technology for the production of silicon single crystals used in the manufacture of integrated circuits is to use the former Czochralski method in which a single crystal of silicon is pulled up from a melt of silicon by using a seed crystal.
In a typical apparatus conventionally used for the Czochralski single crystal growing of semiconductor silicon illustrated in FIG. 3 of the accompanying drawing by a vertical cross section, the quartz glass-made crucible 3 containing the melt of silicon 4 is placed in a metal-made housing 1 of the crystal-growing chamber and the atmosphere inside the chamber 1 is filled with a flowing stream of argon gas shown by the flow lines 12, 13, 14, 16, 17 from the upper conduit 5 on the housing 1 toward the exhaust discharge ports 11 at the bottom of the housing 1. Therefore, the atmosphere of the crystal growing chamber inside the metal-made housing 1 should ideally consist of high-purity argon gas. The fact is that the atmospheric argon gas of the crystal growing chamber is contaminated with various kinds of contaminants including silicon monoxide formed by the reaction of the quartz glass-made crucible 3 and the melt of silicon 4, moisture and oxygen desorbed from the graphite-made parts and components such as the graphite-made receptacle 2, graphite-made heater elements 6, heat-insulators 7 made of a felt of graphite fibers and the like as well as carbon monoxide and carbon dioxide formed by reaction of the moisture and oxygen with the graphite surface heated at a high temperature, e.g., exposed surfaces of graphite-made heater elements 6 and graphite-made receptacle 2, silicon monoxide formed by the reaction of the moisture and oxygen with the melt of silicon 4, carbon monoxide and/or silicon monoxide formed by the reaction of the silicon dioxide of the crucible 3 with the graphite-made receptacle 2 in contact therewith, and so on. Contamination of the atmospheric argon gas with these contaminants is very detrimental to the crystalline structure of the single crystal and increase of the carbon content in the single crystal as grown. Although carbon is an element belonging to the same group as silicon in the Periodic Table and is considered to be electrically inactive, contamination of a silicon single crystal with carbon in fact causes degradation of the electrical properties of semiconductor devices manufactured from such a contaminated single crystal. When the content of carbon in a semiconductor silicon single crystal exceeds 0.5 ppm atomic, for example, the semiconductor devices prepared from such a single crystal may suffer from a decrease in the breakdown voltage of the p-n junction. Accordingly, it is an important technical problem to decrease the carbon content in a semiconductor silicon single crystal used as a material of semiconductor integrated circuits.
The sources of the oxygen and moisture as the contaminants contained in the argon gas of the crystal-growing atmosphere inside the metal-made housing 1 are not limited to the graphite-made structures above mentioned. In the preparatory stage before the start of the Czochralski single crystal growing of a semiconductor silicon from a melt of silicon, the graphite-made parts should be first subjected to a bake-out treatment for a length of time at a higher temperature than in the following actual crystal pulling-up process with the object being to refine the graphite materials by removing the absorbed moisture and oxygen. A problem in this prepartory work is that the bake-out treatment cannot economically be done successfully due to the slowness of desorption from the inside of the graphite materials. Besides, even argon gas of the highest-purity grade available is not absolutely free from various impurities and further the gases initially present in the nooks and recesses in the crystal-growing chamber may more or less remain therein and not be replaced with the argon gas. Therefore, it is not rare that the argon gas of the crystal growing atmosphere is contaminated with moisture and oxygen as problem causing contaminants which cause disorders or other adverse effects in the crystalline structure of the silicon single crystal grown in the atomosphere.
The gaseous silicon monoxide vaporized mainly from the melt of silicon 4 in the crucible 3 is carried away and brought upwardly by the argon gas stream flowing along the complicated flow lines 16, 17 and deposited in the form of a laminar or bulky deposit 18, 19 on the upper periphery of the crucible 3 or the lower surface of the ceiling part of the metal-made housing 1 at a relatively low temperature. The deposite 18, 19 of silicon monoxide once formed in the above described manner may sometimes subsequently fall down to the surface of the melt of silicon 4 and, before being dissolved therein, reach the solid-liquid interface of the growing single crystal 9 by being carried by the surface flow of the melt of silicon 4 which is caused by the convection current or the rotation of the crucible 3, resulting in disorders in the crystalline structure of the single crystal 9.
Moreover, the stream lines of the argon gas flowing in the crystal-growing atmosphere are rarely so simple as schematically expressed by the flow lines 12, 13, 14 in FIG. 3 during the progress of crystal pulling-up but usually involve complicated turbulent flows as shown by the flow lines 15, 16, 17 and 21 to further enhance deposition of silicon monoxide.
It has been proposed in Japanese Patent Kokai 55-113695 as a way to prevent falling of dust of the silicon monoxide from the deposits down to the surface of the melt of silicon 4 that a protector be provided inside the crystal-growing chamber of the apparatus to cover the crucible 3, heater elements 6 and heat-insulating members 7. Alternatively, Japanese Patent Kokai 57-123890 discloses an apparatus in which an orificed gas-flow rectifying baffle is provided close to the quartz glass crucible which serves to rectify the gas flow to promote discharge of the silcon monoxide.
These prior art improvements in the structure of the crystal pulling-up apparatuses are not satisfactory. Accordingly, it is eagerly desired to develop an improved apparatus for single crystal pulling-up by the Czochralski method for the preparation of single crystals of semiconductor silicon which can be operated without the above described problems due to the deposition of silicon monoxide.
It is also known that, when electronic devices constituted by integrated circuits are formed on a wafer of a silicon single crystal in a high density of integration in a process involving a step of thermal oxidation, line defects, referred to as OISFs hereinafter, are induced by the thermal oxidation and act to greatly degrade the performance of the electronic devices together with other microscopic crystalline defects resulting in a decrease in the yield of acceptable products, while control of the OISFs is almost impossible in the conventional apparatuses. Although no clear understanding has yet been obtained of the mechanism leading to the occurrence of OISFs, it is known that OISFs can be classified into two types depending on the modes of occurrence thereof into type A OISFs and type B OISFs. The type A OISFs occur in the outer peripheral portion of about 5 mm width around the single crystal. The distribution density of the type A OISFs in the defective region usually exceeds 20,000/cm.sup.2 on an average with a clear demarcation with the region inside the single crystal free from the type A OISFs.
On the other hand, the type B OISFs are distributed in a density equivalent to or lower than that of the type A OISFs throughout the region from the axis of the single crystal to just inside of the region of the type A OISFs. Increase of the pulling-up velocity of the single crystal is a way to prevent occurrence of the type A OISFs but no effective means has yet been developed to prevent occurrence of the type B OISFs.